Effect of multicomponent modifier on microstructure and mechanical properties of high Ni–Cr–Mo cast iron

Abstract

The microstructure and mechanical properties of high Ni–Cr–Mo indefinite chilled cast iron with the addition of a newly developed multicomponent modifier consisting of mixed rare earths, Si–Ca alloy and Bi–Sb alloy have been investigated through optical microscopy, X-ray diffraction and scanning electron microscopy, along with hardness, impact toughness and wear resistance measurements. After the addition of the modifier, the grain sizes of the primary austenite and eutectic carbides are found to be greatly refined, and the typically highly continuous net-like carbides become less interconnected but rather appear more blocky shaped. Such microstructure changes lead to mechanical property improvement in the cast specimen, with its hardness increased from 43 to 50 HRC, impact toughness from 6·3 to 7·8 J cm^?2 and ?20% increase in abrasive wear resistance.     

Modifying high Cr–Mn cast iron with boron and rare earth–Si alloy

Abstract

By modifying 13Cr–4Mn (wt-%) white cast iron with boron and rare earth (RE)–Si complex, the carbide morphology of the iron can be changed from interconnected, coarse clusters of rods into a parallel distribution of isolated, fine rods, and the impact toughness of the iron can reach 6–7 × 10⁴ J m^?2. In a pin wear test, the relative abrasion resistance of the iron is 1·01 and in a repeated impact abrasive wear test it is 0·95, in comparison with 15Cr–3Mo cast iron. Thus, it is stated that modifying high Cr–Mn cast iron with boron and RE–Si complex is very cost effective, and has almost the same abrasion resistance, when compared with 15Cr–3Mo cast iron.

MST/957     

Properties of ductile cast iron nodularised with multiple calcium–magnesium based master alloy

Abstract

Austempered ductile iron (ADI) is gradually replacing many fabricated and forged steel components in engineering applications. One of its advantages is the combination of good castability, machinability, and mechanical properties with significant savings in cost and weight compared with equivalent steel components. A problem in the production of ADI is the use of expensive and dangerously reactive magnesium as a graphite nodulariser. There is a need to find cheaper, safer, and equally effective substitutes. Results of an investigation of the effectiveness of a multiple calcium–magnesium based master alloy nodulariser and the properties of the ductile iron and ADI produced are reported. Up to 96% nodularity could be obtained using a special Ca–CaC₂–Mg master alloy, compared with 98% using magnesium alone. The mechanical properties were also comparable.     

Structure characterization of a laser-processed Al–Mo alloy

The surface structure of a laser-processed Al–Mo alloy has been characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffractory (XRD). The alloy was prepared by first laser alloying a mixture of Al and Mo powders into an Al substrate and then laser remelting the alloyed surface. Following the first laser alloying process, the needle-like equilibrium phases (Al5Mo(h) and Al5Mo(r)) are formed with a broad size ranges and distribute inhomogeneously in the -Al solid solution matrix. This coarse structure is replaced by a finer, uniform dispersion of dendrites after the subsequent laser remelting. Four basic types of solid states precipitates are observed: (1) irregularly shaped particles constructing the dendrites and having a nearly Al5Mo stoichiometry; (2) needle-like particles which is the Al5Mo (r) phase; (3) Faceted particles having a cubic structure with a stoichiometry close to Al7Mo; (4) tiny, equi-axed particles, with a rather narrow particle size distribution and a cubic structure. © 1998 Chapman & Hall     

Austempering of an Mn–Mo–Cu alloyed ductile iron Part 2 – Structure–mechanical property relationships

Abstract

Ultimate tensile strength, 0·2% proof strength, elongation, and impact energy measurements are reported for an alloyed ductile iron of composition (wt-%) Fe–3·49C–2·33Si–0·42Mn–0·25Cu–0·23Mo–0·035Mg for austempering temperatures of 400, 375, and 350°C and a range of austempering times after austenitising at 920°C for 120 min. The ADI ASTM A897M:1990 standard is satisfied for an austempering temperature of 350°C but not at 375 or 400°C. This behaviour is discussed in terms of the influence of the unreacted austenite volume from the stage I austenitising reaction and the carbide product of the stage II austenitising reaction on the ductility. The present findings are predicted by the processing windows determined from the austempering kinetics.

MST/3393     

Mechanical properties of sinter-hardened Cr–Si–Ni–Mo based steel foam

Highly porous sinter-hardenable Cr–Si–Ni–Mo based steel foam for automotive applications was produced by space holder method. Steel powders were mixed with binder (polyvinylalcohol) and space holder (carbamide), and compacted. Carbamide in the green compacts was removed by water leaching at room temperature. The green specimens were then sintered at temperatures between 1100 °C and 1250 °C for sintering times of 15, 30 and 45 min. In addition, the steel foams were sinter-hardened to enhance mechanical properties. Sinter-hardening combines sintering and heat treatment in one step by increasing the post-sintering cooling rate. This reduces the cost of operation and makes powder metallurgy more competitive. Effects of sinter-hardening process parameters on compressive strength, Young’s modulus, hardness and energy absorption of the steel foams were investigated.     

“The implication of rheological principles for characterization of semi-solid Al–Si cast billets”

The main parameter required to characterize the rheological behavior of semi-solid metal, SSM, slurries is the “viscosity”. Since the viscosity depends on the microstructural characteristics of the SSM slurries and it is also a measure of their rheological behavior, viscometry may be employed to study the rheology and structure of SSM alloys. This is much faster and less expensive microstructural characterization method than quantitative metallography which is a time consuming operation and requires a highly skilled operator. Furthermore, it may be used as an on-line quality check in production of rheo-billets used as feedstock for near net shape manufacturing routes. Al-Si 356 foundry alloy with different morphologies of primary α-Al particles was tested at different initial applied pressures of 5–11 KPa, using parallel plate compression viscometery. The resulting strain–time graphs were further treated mathematically to calculate the viscosity of SSM billets. The viscosity was then attributed to the microstructure and primary phase flow during compression. It was shown that the dendritic primary α-Al structure has the highest viscosity number which is almost three orders of magnitude greater than those for a globular morphology. Such difference reduces to one order of magnitude when the rosette morphology is compared to that of globular structure.This study has shown the validity and reliability of the “parallel plate compression viscometry” method in characterizing the microstructural evolution of rheocast SSM billets and highlights the correlation between the viscosity numbers and the resulting microstructures cast at different pouring temperatures.Prof. Ghomashchi is now director of Advanced Materials and Processing Research Institute,

---

Development a multi-layer perceptron artificial neural network model to estimate the Vickers hardness of Mn–Ni–Cu–Mo austempered ductile iron

The hardness of austempered ductile irons is relative to its microstructure, strength, ductility, machinability and wear resistance properties. Therefore, hardness measurement can be used as a simple tool to control the heat treatment, chemical composition and mechanical properties of ADI parts during the production process. The aim of this study is to develop an Artificial Neural Network (ANN) model for estimating the Vickers hardness of ADIs after austempering treatment. A Multi-Layer Perceptron model (MLP–ANN) was used with Mo%, Cu%, austempering time and temperature as inputs and the Vickers hardness of samples after austempering as the output of the model. A variety of samples were prepared in different conditions of chemical composition and heat treatment cycle. The obtained experimental results were used for training the neural network. Efficiency test of the model showed reasonably good agreement between experimental and numerical results, so the synthesized ANN model can estimate the hardness of the castings with a small error in the range of the experimental results standard deviation.     

Factors affecting eutectic solidification of Cr–Ni (–Si–Mn) white cast irons

Abstract

The crystal structure and morphology of eutectic carbides are known to strongly determine the mechanical and tribological properties of Cr–Ni white cast irons. In an effort to improve these properties, investigators at the US Bureau of Mines have studied the effects of alloying additions of 0·0–1·8%Si, 0·0–6·7%Ni, and 0·0–3·2%Mn (all wt-%) and solidification rates of from 1·0 to ～500 K min^?1 on hypoeutectic irons containing ～3%C and ～8%Cr. The structure and morphology of the eutectic carbides formed were identified using electron microprobe analysis, X-ray diffraction, and scanning electron and optical microscopy. Differential thermal analysis was used to study the effects of alloying additions on the solidification reactions. The results show that these irons can have carbide structures consisting of (Fe, Cr)₃C or (Fe, Cr)₇C₃ or both. These observations are explained in terms of the effects of Si, Ni, and Mn on the liquidus surface of the metastable Fe–Cr–C phase diagram.

MST/1288     

Compressive and impression creep behavior of a cast Mg–Al–Zn–Si alloy

Creep behavior of an Mg–6Al–1Zn–0.7Si cast alloy was investigated by compression and impression creep test methods in order to evaluate the correspondence of impression creep results and creep mechanisms with conventional compression test. All creep tests were carried out in the temperature range 423–523 K and under normal stresses in the range 50–300 MPa for the compression creep and 150–650 MPa for impression creep tests. The microstructure of the AZ61–0.7Si alloy consists of β-Mg₁₇Al₁₂ and Mg₂Si intermetallic phases in the α-Mg matrix. The softening of the former at high temperatures is compensated by the strengthening effect of the latter, which acts as a barrier opposing recovery processes. The impression results were in good agreement with those of the conventional compressive creep tests. The creep behavior can be divided into two stress regimes, with a change from the low-stress regime to the high-stress regime occurring, depending on the test temperature, around 0.009 < (σ/G) < 0.015 and 0.021 < (σ_imp/G) < 0.033 for the compressive and impression creep tests, respectively. Based on the steady-state power-law creep relationship, the stress exponents of about 4–5 and 10–12 were obtained at low and high stresses, respectively. The low-stress regime activation energies of about 90 kJ mol⁻¹, which are close to that for dislocation pipe diffusion in the Mg, and stress exponents in the range of 4–5 suggest that the operative creep mechanism is pipe-diffusion-controlled dislocation viscous glide. This behavior is in contrast to the high-stress regime, in which the stress exponents of 10–12 and activation energies of about 141 kJ mol⁻¹ are indicative of a dislocation climb mechanism similar to those noted in dispersion strengthening mechanisms.     

Simulation of material flow in friction stir processing of a cast Al–Si alloy

The objective of the current paper is using DEFORM-3D software to develop a 3-D Lagrangian incremental finite element method (FEM) simulation of friction stir processing (FSP). The developed simulation allows prediction of the defect types, temperature distribution, effective plastic strain, and especially material flow in the weld zone. Three-dimensional results of the material flow patterns in the center, advancing and retreating sides were extracted using the point tracking. The results reveal that the main part of the material flow occurs near the top surface and at the advancing side (AS). Material near the top surface was stretched to the advancing side resulting in a non-symmetrical shape of the stir zone (SZ). Furthermore, macrostructure and temperature rise were experimentally acquired to evaluate the accuracy of the developed simulation. The comparison shows that the stir zone shape, defect types, powder agglomeration, and temperature rise, which were predicted by simulation, are in good agreement with the corresponding experimental results.     

Simple expressions to estimate the Manson–Coffin curves of ductile cast irons

Due to the possibility of improving and adjusting their mechanical properties using austempering heat treatments, ductile cast irons are very attractive materials for structural applications. However, for this class of materials, very few fatigue data are available for designers in the International Standards. Therefore, at least in the preliminary design phase, simple expressions to estimate the fatigue properties of these materials, taking advantage of their static properties, can be useful for designers. In the recent literature, a simple method was proposed to estimate the strain-life curve of steels based only on the Brinell hardness and the elastic modulus of the material. In this paper, the possibility of extending this approach to ductile cast irons is discussed based on a set of more than 130 fatigue data obtained by the author on ferritic, pearlitic, isothermed and austempered ductile irons.     

Thermodynamic characterization of Cu–Cr system

Abstract

The Cu–Cr phase diagram has been characterized thermodynamically and the subsequently calculated phase diagram is in good agreement with experimental results. The results show that the equilibrium solubility of chromium in copper becomes negligible below 600°C and confirm that the chromium solidus is retrograde.

MST/650     

The fatigue damage development in a cast Al–Si–Cu alloy

The experimental identification of fatigue damage mechanisms and evaluation of their development rate, based on changes in material respond on cycle loading, has been presented in the work. The research has been conducted on hyper-eutectic cast alloy AlSi8Cu3. The microstructure and fracture analyses were performed. The high cycle fatigue tests were conducted with frequency of 20 Hz under constant nominal stress amplitude with monitoring the strain response of material during the test. The ratcheting was found as the main mechanism of the fatigue damage. It was established that the linear fatigue accumulation law should not be used for fatigue life prediction in case of the tested cast aluminum alloy.     

Investigation of microstructure and mechanical properties of steel fibre–cast iron composites

Abstract

The aim of the present experimental study was to investigate improvement of the toughness and strength of grey cast iron by reinforcing with steel fibres. The carbon content of the steel fibres was chosen to be sufficiently low that graphite flakes behaving as cracks were removed by carbon diffusion from the cast iron to the steel fibres during the solidification and cooling stages. To produce a graphite free matrix, steel fibres with optimum carbon content were used and the reinforced composite structure was cast under controlled casting conditions and fibre orientation. Three point bend test specimens were manufactured from steel fibre reinforced and unreinforced flake graphite cast iron and then normalising heat treatments were applied to the specimens at temperatures of 800 and 850°C. The fracture toughness and strength properties of the steel fibre reinforced material were found to be much better than those of unreinforced cast iron. The microstructures of the composite at the fibre–matrix transition zone were examined.     

Effect of deep cryogenic treatment on the microstructure and wear performance of Cr–Mn–Cu white cast iron grinding media

The phase transformation and grinding wear behavior of Cr–Mn–Cu white cast irons subjected to destabilization treatment followed by air cooling or deep cryogenic treatment were studied as a part of the development program of substitute alloys for existing costly wear resistant alloys. The microstructural evolution during heat treatment and the consequent improvement in grinding wear performance were evaluated with optical and scanning electron microscopy, X-ray diffraction analysis, bulk hardness, impact toughness and corrosion rate measurements, laboratory ball mill grinding wear test etc. The deep cryogenic treatment has a significant effect in minimizing the retained austenite content and converts it to martensite embedded with fine M₇C₃ alloy carbides. The cumulative wear losses in cryotreated alloys are lesser than those with conventionally destabilized alloys followed by air cooling both in wet and dry grinding conditions. The cryotreated Cr–Mn–Cu irons exhibit comparable wear performance to high chromium irons.     

Phase characterization in as-cast F-75 Co–Cr–Mo–C alloy

Microstructural characterization of a cast acetabulum of ASTM F-75 alloy has been carried out in order to clarify conflicting reports from the literature. The present investigation revealed that although sigma (σ) and M₂₃C₆ carbide were the only secondary phases formed in the face centered cubic cobalt-base alpha matrix (Co-α), as identified by X-ray diffraction, the observed microstructure was quite complex. Scanning and transmission electron microscopy indicated the presence of coarse and fine lamellar cellular colonies, grain boundary film carbide, and different types of coarse blocky particles, including single-phase σ, dual-phase σ/M₂₃C₆, a binary eutectic comprised of σ and Co-α phases, and a three-phase feature comprising the binary eutectic and solid state formed M₂₃C₆. The carbide has probably formed during cooling from casting due to σ metastability. While it is proposed that the some lamellar cellular colonies were formed by discontinuous precipitation, it is not clear whether all lamellar structures present in the as-cast alloy occurred due to the same mechanism. The results obtained for the tensile properties are discussed in view of the observed microstructure.     

Microstructure and properties of Ti–Nb–V–Mo-alloyed high chromium cast iron

The correlations of microstructure, hardness and fracture toughness of high chromium cast iron with the addition of alloys (titanium, vanadium, niobium and molybdenum) were investigated. The results indicated that the as-cast microstructure changed from hypereutectic, eutectic to hypoeutectic with the increase of alloy contents. Mo dissolved in austenite and increased the hardness by solid solution strengthening. TiC and NbC mainly existed in austenite and impeded the austenite dendrite development. V existed in multicomponent systems in forms of V alloy compounds (VCrFe₈ and VCr₂C₂). With the increase of alloy additions, carbides size changed gradually from refinement to coarseness, hardness and impact toughness were increased and then decreased. Compared with the fracture toughness (6 J/cm²) and hardness (50·8HRC) without any alloy addition, the toughness and hardness at 0·60 V–0·60Ti–0·60Nb–0·35Mo (wt%) additions were improved and achieved to 11 J/cm² and 58·9HRC, respectively. The synergistic roles of Ti, Nb, V and Mo influenced the solidification behaviour of alloy. The refinement of microstructure and improvement of carbides morphologies, size and distribution improved the impact toughness.     

Effect of austenitising temperature on tensile properties of Cu–Ni austempered ductile iron

Abstract

The effect of austenitising temperature on the microstructure, mechanical properties, and dimensional stability of a spheroidal graphite iron containing copper and nickel has been investigated. It was found that as the temperature increased the amount of carbon taken into solution by the austenite increased thus reducing the driving force of the original austenite to bainitic ferrite and high carbon austenite. As a consequence, the amount of retained austenite increased, but its stability decreased. This placed an upper limit on the austenitising temperature and on the amount of retained austenite permissible. All properties other than hardness showed maximum values after austenitising at 900°C. It was also found that increasing the solution treatment temperature increased the dimensional stability.

MST/1116     

Microstructural changes during annealing of cast Ni–Cr–Hf alloys

Abstract

In order to gain knowledge of the nickel–rich corner of the Ni–Cr–Hf phase diagram, microstructures of cast alloys and their evolution towards equilibrium conditions have been followed by optical and scanning electron microscopy, microprobe analysis, and quantitative metallography. The evolution of as–cast microstructures involves the development of a Ni₅Hf plate–like precipitate in two–phase alloys, and various morphological changes in three–phase alloys. As–cast structures are compared with those obtained by arc melting. Direct processing of the scanning electron microscope image has proved to be a valuable tool for microstructural characterization of the transforming phases.

MST/349